Study On The Pyrolysis Characteristics Of Vitex Negundo Var In N₂/CO₂ Atmosphere

Vitex negundo var is widely distributed in Beipiao mountain area of Liaoning Province,which is a kind of Bio Energy Petroleum grass and have the characteristics of high oil content and high calorific value.At present,it is still in the stage of undeveloped utilization,and scholars at home and abroad have little research on it,and it has certain development potential.This article takes Vitex negundo var as the research object,and characterizes and analyzes it by thermochemical conversion method.Thermogravimetric test and thermogravimetric infrared(TG-FTIR)technology were used to study its pyrolysis characteristics,pyrolysis gas products and pyrolysis reaction kinetics.At the same time,fixed bed test bench was used to investigate the composition and distribution of experimental temperature on pyrolysis tri-state products in N2 atmosphere,and the study on high-temperature pyrolysis biochar in CO2 atmosphere.Firstly,the physical and chemical properties of the Vitex negundo var samples were analyzed,and the results showed that it had the characteristics of higher volatile content(73.83%),lower ash content(1.65%),higher C,H,O content and trace S content,with the calorific value of 17565.06 kJ/kg,which has good fuel properties;the biochemical composition according to the content is cellulose(40.2%)>hemicellulose(34.6%)>lignin(21.1%).Secondly,TG-FTIR technology was used to conduct pyrolysis of Vitex negundo var under N2 atmosphere and temperature rise rate of 20℃/min,to analyze the characteristics of the pyrolysis process and the precipitation of pyrolysis volatile products,and to compare the pyrolysis characteristics with straw and rice husk under the same thermogravimetric conditions.The results show that the pyrolysis process of Vitex negundo var can be divided into four stages,that is,the drying stage of water loss,the transition stage of preheating solution,the pyrolysis volatilization analysis stage,and the pyrolysis carbonization stage.The main pyrolysis volatile products are small molecule gases,mainly H2O,CH4,CO2 and CO,as well as organic gases such as acids,ketones and phenols.The thermal stability of rice hull and straw was as follows:rice hull>Vitex negundo var>straw.Then the thermogravimetric experiment was carried out on the Vitex negundo var under N2 and CO2 atmospheres at different heating rates to investigate the effects of different heating rates and reaction atmospheres on the pyrolysis process,and the kinetic parameters were determined by the FWO method,Friedman method and CR method.The results show that:with the increase of the heating rate,the pyrolysis under N2 and CO2 atmosphere shows thermal hysteresis,and the thermogravimetric curves are shifted to the high temperature side.The apparent activation energies of the main pyrolysis intervals in N2 and CO2 atmosphere were 190.35 kJ/mol,189.91 kJ/mol,175.71 kJ/mol and 172.16 kJ/mol,respectively,obtained by FWO and Friedman methods.Moreover,the mechanism functions of CR method were determined to satisfy the Avrami-Erofeev equation,which was a mechanism form of random nucleation and subsequent growth,and the reaction series n=4,the integral form is G(α)y[-ln(1-α)]4.Finally,a self-built fixed bed pyrolysis bench was used to test the samples of Vitex negundo var samples,to explore the influence of pyrolysis final temperature on the pyrolysis tristate products under N2 atmosphere and the pyrolysis biochar under CO2 atmosphere at high temperature.Meanwhile,the products were analyzed by GC,GC-MS,FTIR and SEM.The results show that under N2 atmosphere,the volume percentage of CO and H2 gas in condensable pyrolysis gas increases with the increase of the final pyrolysis temperature,CO2 gas decreases on the contrary,CH4 gas peaks at 450℃.The main component of bio-oil at 250℃ is organic acids,and the content of phenolic compounds increases with the increase of pyrolysis final temperature.The main content is guaiacol,catechol,2,6-dimethoxyphenol.With the increase of the pyrolysis temperature of biochar,the alkane bond breaks,the hydroxyl group is released,and gradually aromatizes,the carbon condensation degree increases,at the same time,the surface morphology changes dramatically,and the skeleton collapses to form a porous appearance.The carbon content of pyrolysis biochar in high-temperature pyrolysis under CO2 atmosphere is about 85%,and the Boudouard reaction occurs at 850℃.